1. Field of the Invention
The present invention relates to an inductor fabricated with a dry film resist (DFR) and a cavity formed between an insulating layer and a lower electrode so as to have a high self resonant frequency (SRF) and a high quality factor Q.
2. Description of the Related Art
An inductor has a space limitation in a wireless circuit area. Thus, a spiral inductor drawing a concentric circle in one direction using a pattern is mainly used. The spiral inductor is wound in the same direction to increase a magnetic field. Thus, although the spiral inductor has a small size, the spiral inductor forms an inductance having a large value.
For an integrated circuit used for a wireless circuit, the spiral inductor is applied in a frequency band between 800 MHz and 2400 MHz in various fields.
FIG. 1 is a cross-sectional view of a conventional spiral inductor. Referring to FIG. 1, in an inductor 100, a metal layer 20 that is a lower electrode is stacked on a substrate 10, and an insulating layer 30 is deposited on the metal layer 20. Viaholes 40 and 50 are formed in the insulating layer 30.
An upper electrode 60 that is patterned in a spiral shape is formed on the insulating layer 30. The upper electrode 60 is connected to the lower electrode 20 underneath the insulating layer 30 via the viaholes 40 and 50.
According to the structure of the inductor 100, energy loss occurs due to an eddy current generated through the substrate 10 by the spiral structure of the upper electrode 60 and a resistance component of the substrate 10. Also, a parasitic capacitance component generated by the structures of the upper electrode 60, the insulating layer 30, and the lower electrode 20 may not be neglected.
A quality factor Q indicating the quality of an inductor is obtained by dividing magnetic energy stored in the inductor by lost energy. Energy loss occurs due to resistances of a metal line and a silicon substrate. As a frequency is increased by a skin effect, the resistance of the metal line is increased. Also, energy loss of the silicon substrate is increased with the increase in the frequency. Also, the magnetic energy is decreased with the increase in the frequency. Energy generated by the parasitic capacitance component caused by the shape of the inductor is increased with the increase in the frequency. Thus, the quality factor Q has a maximum value at a specific frequency and then gradually decreases.
Also, the magnetic energy gradually decreases and the energy generated by the parasitic capacitance component increases with the increase in the frequency. Thus, the inductor loses its function but operates as a capacitor so as to generate an SRF.
To solve these problems, an insulating layer is formed of a material having a low dielectric constant to reduce a parasitic capacitance between upper and lower electrodes. Also, the thickness of the insulating layer is increased to reduce the parasitic capacitance between the upper and lower electrodes. However, this has a limit, and energy loss of a substrate is not removed.
In a conventional inductor disclosed in U.S. Pat. No. 5,844,299 and entitled “Integrated Inductor,” a substrate under an inductor is removed to reduce an effect of the substrate. However, the parasitic capacitance between upper and lower electrodes cannot be reduced. Also, to remove the substrate, a cavity is formed, filled with a thick silicon oxide material, and planarized using a method such as lapping and chemical mechanical polishing (CMP). Thus, a process of fabricating the conventional inductor is complicated.
Accordingly, a method of simultaneously removing energy loss due to a substrate and a parasitic capacitance between upper and lower electrodes is required.